Seek an interim order from the Tribunal that he must not alter the common property. In background information for the Tribunal you would include the minutes of the meeting at which approval was sought and declined. Include a copy of any correspondence in which he threatened to proceed or a statement that such a threat was delivered verbally.
You could also seek a separate order that the already altered common property be restored.
An option open to the owner would be to seek an order to give effect to his failed motion to be allowed to move his front door. You can choose whether or not to point that out to the owner. The owner might be able to make a case that it was unreasonable to not be allowed to move the door.